Down-and up-conversion processes in Nd3+/Yb3+ co-doped sodium calcium silicate glasses with concomitant Yb2+ assessment

Novel Nd3+/Yb3+co-doped sodium calcium silicate glasses were prepared by melting quenching method:Spectroscopic study was carried out as a function of doping content by fixing sensitizer(Nd3+) concentration to 0.2 mol% and adjusting activator(Yb3+) from 0 to 1.0 mol%.The energy transfer(ET)mechanisms between Nd3+and Yb3+are discussed based on their energy levels and excitation powerdependence emission intensity.Results show that the presence of Yb2+might be considered for the Nd3+-free and co-doped samples.The ET was confirmed by the down-conversion NIR emission spectra of the doped and co-doped samples under excitation at 808 nm.The mechanisms observed seem to involve only one VIS absorbed photon for each NIR emitted via direct energy transfer between4F3/2of Nd2+and2F5/2of Yb3+compensated by phonon assistance due to energy gap between these levels.The efficiency of ET increases with the ytterbium content up to almost 90% for the sample with 1 mol% of Yb2O3,which was evaluated by lifetime measurements.Up-conversion photoluminescence by exciting trivalent ions of neodymium(808 nm) and ytterbium(975 nm) is reported.The observed up-converted emission bands are related to the 4f-4f transitions of Nd3+and the spin-forbidden 5d-4f transition of Yb2+.Nd3+upconversion emission is observed under 975 nm excitation,presenting an almost quadratic emission dependence with power excitation,which suggests that two laser photons participate in the up-conversion(UC) process,showing that ET occurs by a phonon-assisted energy transfer and cooperative energy transfer.